It's to do with weak muscles in the area of the diaphragm
When you are running for a long time and heavy breathing will cause muscles to get worn out and staring to ache and cramp up.
When running try doing these few tips:
1. When you are running, try not to turn you head often since this and the combination of heavy breathing can cause the muscles to cramp.
2. Keep your shoulder relaxed and keep a good locked elbows at 90 degrees, some times you might look a tit but its better than damage to yourself
3. similar to the first tip, when running dont jerk your chest around, do not swing your arms inwards to your body.
4. Do not inhale or exhale to violently, try to develop your own breathing pattern, the most common for long distance runners like me is 3 paces inhaling, 2 paces exhaling. If your in pain just stop, you will just do damage to yourself.
5. work on your lats and chest muscles since a stronger chest and larger muscles will create a better blood flow and creates less stress on the muscle, you might have the biggest pecks in the world but there are muscles all around the chest and shoulder which need to be worked on aswel.
6. Do Not Forget to do proper stretches, don't just focus on the leg muscles